A battle nearly 40 years in the making is coming to a head at one of Southern California's most iconic beaches, pitting the suits against the people who don't wear any.

Swimsuits, that is.

After decades of looking the other way, officials at San Onofre State Beach in north San Diego County are set to crack down on a clothing-optional stretch of sand where people soak up the sun without fear of tan lines.

And opponents to that effort have filed a lawsuit to stop it.

Citing ongoing complaints from park visitors and the fear of workplace harassment lawsuits from employees, officials say that after Labor Day, they will begin citing skinny dippers who refuse to cover up. New large signs warning that nudity is prohibited have recently sprouted throughout the park, and rangers are telling nude sunbathers that their endless summer is about to end.

"Times have changed," said Rich Haydon, acting superintendent of the California Parks and Recreation Department's Orange Coast district. "The population growth within a two-hour drive of San Onofre has grown tremendously through the years. It can no longer be considered a remote beach."

Angered naturists say they intend to fight the move lying down - in the sand, as hundreds of nude sunbathers do every summer weekend.

"Do you think one or two rangers could cite all those people? No way," said R. Allen Baylis, who heads Friends of San Onofre Beach, a naturist group. "There's going to be no way to effectively enforce this policy."

Haydon responded with a chuckle. "It will be enforced," he said. "We've already been in discussion with other law enforcement agencies."

The Naturist Action Committee filed a lawsuit in Orange County Superior Court on Wednesday against the state agency. The suit claims the agency should have sought public comment before banning nudity at the 1,000-foot beach. A hearing is set for Aug. 20.

San Onofre's surf breaks are internationally known, in particular the perfectly shaped lines at Trestles and the easygoing 1960s time warp at the longboarders' hub known as Old Man's Beach.

Naturists worldwide know San Onofre for Trail 6, a dirt path that snakes down from sandstone bluffs to the beach's southern end, where it meets the Camp Pendleton Marine Corps Base.

When President Nixon transferred part of the base to the state for use as a park, he told a reporter, "This is a great sunning beach."

At issue is a murky combination of regulations and policies that park rangers have used for years to deal with nudity.

State law forbids nudity in state parks "except in authorized areas set aside for that purpose." But there are no such areas. In the late 1970s, Russell Cahill, then-director of the state parks department, proposed establishing "clothing optional" areas but dropped the idea in the face of opposition and concerns over the enforcement costs.

Instead, he issued what's become known as the Cahill Policy, under which citations or arrests are made only after a complaint from the public and attempts to "elicit voluntary compliance."

Beachgoers have been baring it all at Trail 6 since the park opened in 1973. Back then, San Onofre was about as isolated as a place could be in Southern California, a strip of sand that was a long drive from a creeping metropolis that had not yet reached it.

"There's a mystique to San Onofre, even today," said Haydon, who first went to work there more than 20 years ago as a seasonal lifeguard. "It's a throwback to what California looked like 100 years ago."

In some respects, though, San Onofre isn't what it was even a decade ago.

The beach had 2.5 million visitors in 2007, up from 1.6 million in 2000. Some of the newcomers are upending San Onofre's cherished informality: Bonfires, beers and longboards evoked a California beach culture a generation ago that exists today only on Super 8 film.

Until last summer, San Onofre was the rare Southern California beach where alcohol was still legal. But it was banned after an increase in brawls, drunken driving and other alcohol-related offenses.

"People were coming down and taking over the place as a party spot," Haydon said. "Surfers were being harassed. ... It necessitated such an action."

Similarly, the move to place a metaphorical fig leaf on San Onofre's nude beach is in large part driven by increasing reports of lewd behavior, Haydon said.